A fuel cell system mounted on a fuel cell vehicle generates electricity through electrochemical reaction and produces water in association with the electrochemical reaction. The fuel cell (stack) of the fuel cell system is usually configured by stacking many minimum structural units, referred to as cells. In a conventional solid polymer fuel cell, as shown in FIG. 7, each of the cells 201 is configured such that catalyst layers 206 and 207 for activation of the reaction, between which an electrolyte membrane 208 selectively allowing permeation of hydrogen ions is sandwiched, are provided between diffusion layers 204 and 205, and such that the diffusion layers 204 and 205 are sandwiched between an anode electrode 202 and a cathode electrode 203 for respectively supplying hydrogen and air (oxygen).
Hydrogen molecules supplied to the anode electrode 202 turn into active hydrogen atoms in the catalyst layer 206 provided on the surface of the electrolyte membrane 208 on the side of the anode electrode 202, and further turn into hydrogen ions to release electrons. The reaction indicated by (1) in FIG. 7 is expressed by following Chemical Formula 1.H2→2H++2e−  (Formula 1)
Hydrogen ions generated in Chemical Formula 1 are moved through the electrolyte membrane 208 from the side of the anode electrode 202 to the side of the cathode electrode 203 together with moisture contained in the electrolyte membrane 208, while electrons are moved to the side of the cathode electrode 203 through an external circuit 209. By the movement of electrons, current flows through a load (for example, a traveling motor of a vehicle) 210 interposed in the external circuit 209.
On the other hand, oxygen molecules in the air supplied to the cathode electrode 203 receive, in the catalyst layer 207, electrons supplied from the external circuit 209, so as to turn into oxygen ions, and the oxygen ions combine with the hydrogen ions having moved through the electrolyte membrane 208, so as to form water. The reaction indicated by (2) in FIG. 7 is expressed by following Chemical Formula 2.½O2+2H++2e−→H2O  (Formula 2)
A part of water formed in this way moves from the cathode electrode 203 to the anode electrode 202 by concentration diffusion.
In the above-described chemical reaction, various losses, such as a resistance overvoltage resulting from the electric resistance of the electrolyte membrane 208 and the electrode, an activation overvoltage required for causing the electrochemical reaction of hydrogen and oxygen, and a diffusion overvoltage required for causing the movement of hydrogen and oxygen through the diffusion layers 204 and 205, are generated in the inside of the cell 201, and the waste heat generated due to the losses needs to be removed.
FIG. 8 shows a general configuration of a water-cooled fuel cell system equipped with the cell 201 and used for a conventional fuel cell vehicle. In a fuel cell system 301 shown in FIG. 8, a fuel cell 302 configured by stacking many cells that are the minimum structural units described above is provided, and power generation is performed by the many cells stacked in the fuel cell 302 in such a manner that compressed hydrogen gas stored in a high pressure hydrogen tank 303 is introduced from an anode intake passage 304 into an anode intake portion 306 of the fuel cell 302 via a pressure reducing valve 305, and that, on the other hand, the outside air drawn into a cathode intake passage 308 through a filter 307 is compressed by a compressor 309, so as to be introduced into a cathode intake portion 310 of the fuel cell 302.
After a part of moisture in cathode exhaust gas discharged from a cathode exhaust portion 311 of the fuel cell 302 into a cathode exhaust passage 312 is separated by a gas-liquid separator 313, the cathode exhaust gas is discharged to the outside air via a backpressure valve 314 used for the pressure control of the cathode system. Similarly, anode exhaust gas discharged from an anode exhaust portion 315 of the fuel cell 302 into an anode exhaust passage 316 is made to pass through a gas-liquid separator 317, and is mixed into the cathode exhaust gas through the anode exhaust passage 316 connected in the middle of the cathode exhaust passage 312 via a purge valve 318.
The exhaust amount (flow rate) of purge hydrogen from the anode exhaust portion 315 is sufficiently small as compared with the amount of cathode exhaust gas. For this reason, the anode purge hydrogen can be discharged to the outside air after the concentration of the anode purge hydrogen is reduced by the cathode exhaust gas to at most the flammable lower limit concentration of 4%. Note that there are also some systems in which, in order to improve the utilization of hydrogen, the anode exhaust gas is recirculated to the anode intake portion 306 by using a hydrogen pump 320 interposed in an anode return path 319 connecting the anode exhaust passage 316 to the anode intake portion 306.
Here, a cooling system 321 of the water cooled fuel cell system 301 is described. In a cooling water introduction passage 322 of the cooling loop of the cooling system 321, a water pump 323 is provided at a stage preceding or subsequent to the fuel cell 302, so as to pressure-feed cooling water to a radiator 324. After cooling the fuel cell 302, the cooling water is subjected to heat exchange in the radiator 324, and is then again returned to the fuel cell 302 through a cooling water lead-out passage 325 of the cooling loop.
A heating apparatus 326 is provided in the cooling system 321. The heating apparatus 326 includes a heating passage 327 for connecting the cooling water introduction passage 322 to the cooling water lead-out passage 325, and also includes a heater core 329 for heating the interior of the vehicle compartment, and a regulating valve 328 which are connected in series with each other by the heating passage 327, and which are provided in parallel with the radiator 324. When the interior of the vehicle compartment needs to be heated, the heating apparatus 326 performs heating by opening the regulating valve 328 to supply high temperature cooling water to the heater core 329 and by driving a fan 330 for blowing air. However, the amount of waste heat of the fuel cell 302 is very small as compared with the amount of heat generated by an engine, and hence the other auxiliary heat source, such as an electric heater, is generally used in addition to the heater core 329.
As described above, the water-cooled fuel cell vehicle system 301 is provided with various auxiliary units including the compressor 309 for compressing the introduced air, in order to increase the output power density of the fuel cell 302. For this reason, in the water-cooled fuel cell vehicle system 301, the system is complicated, and the size, weight and cost of the system are increased. On the other hand, there is an air-cooled fuel cell system in which the system is simplified in such a manner that auxiliary units, such as a compressor, are eliminated as much as possible, and that an air cooling system is adopted for cooling the fuel cell.
As shown in FIG. 9, in an air-cooled fuel cell system 401 equipped with a fuel cell 402 configured by stacking many cells that are minimum structural units as described above, a compressed hydrogen gas stored in a high pressure hydrogen tank 403 is introduced into an anode intake portion 406 of the fuel cell 402 after the pressure of the compressed hydrogen gas is reduced by a pressure reducing valve 405 interposed in an anode intake passage 404. On the other hand, in the fuel cell system 401, the high-pressure compressor for supplying cathode intake air, which is provided in the water-cooled fuel cell vehicle system, is not generally provided, and the outside air drawn into a cathode intake passage 408 through a filter 407 is supplied to a cathode intake portion 410 of the fuel cell 402 by a low-pressure air supply fan (blower) 409.
The air supplied to the cathode intake portion 410 is not only used as the reaction gas reacting with hydrogen in power generation reaction in the many cells stacked in the fuel cell 402 but also plays a role of removing the waste heat in the fuel cell 402 so as to cool the fuel cell 402. The air after reaction with hydrogen and the air after cooling the fuel cell 402 are discharged from a cathode exhaust portion 411 of the fuel cell 402 to a cathode exhaust passage 412, so as to be discharged to the outside air. The anode exhaust gas discharged from an anode exhaust portion 413 of the fuel cell 402 to an anode exhaust passage 414 is mixed into the cathode exhaust gas through the anode exhaust passage 414 connected in the middle of the cathode exhaust passage 412 via a purge valve 415. When the hydrogen gas on the anode side is purged, the exhaust hydrogen gas is diluted to the flammable lower limit concentration or less by the cathode side exhaust gas, so as to be discharged to the outside.
The air-cooled fuel cell system 401 is not equipped with the cooling system 321 as provided in the water-cooled fuel cell system 301 shown in FIG. 8, and hence cannot realize a heating apparatus similar to the heating apparatus 326 in the water-cooled fuel cell system 301.
As examples of a heating apparatus of a conventional fuel cell vehicle, there have been proposed a heating apparatus in which cathode exhaust gas of a fuel cell is directly discharged into the vehicle compartment (Patent Literature 1), a heating apparatus in which cathode exhaust gas of a fuel cell is directly discharged into a vehicle compartment and is also led to a heat exchanger of the heating apparatus for heating the vehicle compartment (Patent Literature 2), and a heating apparatus in which cathode exhaust gas of a fuel cell is led to a heat exchanger of the heating apparatus for heating the vehicle compartment (Patent Literature 3).